1. Field of the Invention
This invention relates in general to storage systems, and more particularly to a method and apparatus for predicting write failure resulting from flying height modulation and initiating re-writing of data upon occurrence of the predicted write failure.
2. Description of Related Art
Modern computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. In such devices, read-write heads are used to write data on or read data from an adjacently rotating hard or flexible disk.
Existing magnetic storage systems use magnetoresistive (MR) heads to read data from magnetic media and to write data onto magnetic media. MR disk drives use a rotatable disk with concentric data tracks containing the user data, a read/write head that may include an inductive write head and an MR read head for writing and reading data on the various tracks, a data readback and detection channel coupled to the MR head for processing the data magnetically recorded on the disk, an actuator connected to a carrier for the head for moving the head to the desired data track and maintaining it over the track centerline during read or write operations.
There are typically a plurality of disks stacked on a hub that is rotated by a disk drive spindle motor. A housing supports the drive motor and head actuator and surrounds the head and disk to provide a substantially sealed environment for the head-disk interface. The head carrier is typically an air-bearing slider that rides on a bearing of air above the disk surface when the disk is rotating at its operational speed. The slider is maintained in very close proximity to the disk surface by a relatively fragile suspension that connects the slider to the actuator. The spacing between the slider and the disk surface is called the flying height and its precise value is critical to the proper function of the reading and writing process.
The inductive write head and MR read head are patterned on the trailing end of the slider, which is the portion of the slider that flies closest to the disk surface. The slider is either biased toward the disk surface by a small spring force from the suspension, or is “self-loaded” to the disk surface by means of a “negative-pressure” air-bearing surface on the slider.
The MR sensor detects magnetic field signals through the resistance changes of a magnetoresistive element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the element. MR sensors have application in magnetic recording systems because recorded data can be read from a magnetic medium when the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in an MR read head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage. The conventional MR sensor used in magnetic recording systems operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the element resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. A particularly useful application of GMR is a sandwich structure comprising two essentially uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is “pinned”, and thus prevented from rotating in the presence of an external magnetic field. This type of MR sensor is called a “spin valve” sensor. U.S. Pat. Nos. 5,159,513 and 5,206,590, commonly assigned to the assignee of the present invention, describe MR spin valve sensors for use as MR read heads in magnetic recording data storage systems.
The read-write heads have been designed so that they will fly over the surface of the rotating disk at a very small, though theoretically constant distance above the disk. The separation between the read-write head and the disk is called the flying height, and is maintained by a film of air. The flying height is critical to proper function during reading and writing. If the flying height is too high during read, the read head will not be able to resolve the fine detail of the magnetic signal, thereby resulting in undecipherable data. Similarly, if the flying height is too high during a write, the magnetic flux lines that intersect the plane of the disk surface become weaker, thereby leading to loss of resolution. It is known that small solid or liquid contaminants inside the disk drive may collide with the head while it is either reading or writing, temporarily inducing substantial flying height increases. If this occurs during reading, the drive will detect the poor signal and initiate a recovery procedure, e.g., typically simply retrying the read. Because contamination collisions are transient events, the simple read retry is usually successful. However, if the flying height modulates during a write, today's drives do not detect that there is any problem at all. It is only after this poorly written data is read back that a problem is discovered. Poorly written data cannot be deciphered even when read under perfect ideal conditions, so by this time it is too late to recover.
During flight, the head undergoes continuous vibration, pitch and roll as the topography of the disk changes beneath the head. In a conventional hard disk device, heat is produced by the MR head due to its normal current bias. This heat is dissipated by the cooler disk. Because the film of air separating the head and the disk is of a thermally insulating nature, the amount of heat dissipation from the head depends upon flying height. An increase in the flying height due to modulation will cause the temperature to rise in the MR sensor. This increases the resistance of the MR head and can be detected as a low-frequency blip in the voltage output from the MR head. This is the definition of the thermal signal.
If the flying height modulates during the write process, the write head may fail to write the data properly. The modulation will affect the higher frequency component of the write signal more than the low frequency components. Thus, when this poorly written data is read back, the resulting signal will consist of a low-frequency modulation envelope with reduced amplitude in the high frequency signals. This high-frequency dropout will result in unrecoverable media errors when the data is attempted to be retrieved.
U.S. Pat. No. 5,751,510 to Smith, et al., which is commonly assigned to the assignee of the present applicant, and which is incorporated by this reference herein, discloses an apparatus and method for reading an information signal from a magnetic storage medium using a magnetoresistive (MR) element, modifying the signal such that a thermal component of the signal representing a thermal response of the MR element is degraded, and altering the modified signal to produce a restored thermal signal substantially representative of the thermal component of the information signal read from the storage medium. The restored thermal signal may be used to detect disk surface defects and topographic variations, and may be utilized for other systemic and diagnostic purposes, including disk surface defect characterization, error correction, and predictive failure analysis. However, U.S. Pat. No. 5,751,510 does not disclose how to determine whether to initiate a re-write operation.
U.S. Pat. No. 6,088,176 to Smith, et al., which is commonly assigned to the assignee of the present applicant, and which is incorporated by this reference herein, discloses separating a thermal signal component and, if present, a magnetic signal component from the information signal. The magnetic signal is processed to remove the influence of the thermal signal component from the magnetic signal. According to U.S. Pat. No. 6,088,176, the magnetic and thermal signal components of a readback signal are respectively extracted and processed so as to linearly correspond to head-to-disk spacing. Head-to-disk spacing using the thermal signal may be used to detect disk surface defects and topographic variations. The thermal signal may be calibrated using a magnetic spacing signal in order to directly measure head-to-disk spacing change. The thermal head-to-disk spacing signal may be utilized for other systemic and diagnostic purposes, including defect characterization, error correction, and predictive failure analysis. However, U.S. Pat. No. 6,088,176 also fails to detect when a flying height modulation occurs during a writing and to determine when to initiate a re-write operation.
It can be seen then that there is a need for a method and apparatus for predicting write failure resulting from flying height modulation and initiating re-writing of data upon occurrence of the predicted write failure.
It can also be seen then that there is a need for a method and apparatus that will detect when such a modulation occurs and force a rewrite of the same data when necessary.